Consequence and Active Use of Free Gas in Hydro Power

This is an industrial PhD project at NTNU and FDB with financial support from The Research Council of Norway. More information about the industrial PhD project can be found in this interview with The Research Council of Norway.

The main objectives of the project are to investigate the effect of free gas in hydro power plants and how injection of gas can be used to reduce unwanted effects like pressure pulsations, noise and vibration, which are typically present at part load operation of Francis turbines. The project will include experiments both in laboratories and power plants and two experiments have already been carried out.

SAFL study

The experiment was carried out in the cavitation tunnel at St. Anthony Falls Laboratory (SAFL) at the University of Minnesota, Minneapolis. We were looking at the effect of gas content on cavitation and tunnel dynamics. Shedding frequency from a cavitating hydrofoil was used as the excitation frequency for the system and the pressure response was measured in three different location in the tunnel loop.

Svorka Visual

Four plexiglass windows have been installed for visual access to the draft tube flow on Svorka power plant (25 MW), operated by Statkraft. Such visual access to the draft tube flow on an operating machine is unique, as we normally only have visual access in laboratory tests of the model scale runner. A visual study of blade outlet cavitation will be combined with cavitation intensity measurements in this experiment.

Figure 1: Cavitating hydrofoil in SAFL cavitation tunnel

Figure 2: Installation of plexi glass windows on draft tube

Figure 3: Cavitation at runner outlet

Flow Control

FDB defines FC as the collection of techniques used for locally improvement of flow conditions around and through components. FC often improves performance at operation points that differs from the design or optimum operation point. The aerospace industries uses a number of such techniques and solutions such as vortex-generation distributed along the span of the wing, and chevrons at the trailing edge of the outer duct exit of the gas turbine engines. The former FC solutions improves lift/drag ratio at high angle-of-attacks conditions during take-off and landing, while the latter mitigate noise. Both being typical FC solutions in that they solve secondary problems, but with negligible impact on normal or main operation of the same airplane.

FC makes up a part of fluid dynamic that enjoys a broad interface between academia and the industry, and mainly because the economic gains using FC especially relates to large or complex industrial flow systems. Generally, the success of FC products relies on superior knowledge of the main flow object, being an airplane or a hydropower turbine, as well as of the FC technology itself, such as vortex generators or injection systems.

FDB actively pursue Flow Technology, and our activities include:

By shooting water tangentially into the hydroturbine draft tube, DiffCon mitigates pressure pulsations at off design operation and allows an expanded allowable operation range of the same unit. FDB’s Håkon Francke completed his PhD where he focused on design optimization over the parameter-set used for describing the DiffCon- system.

Vortex generators for mitigation load oscillations due to cavitation and free gas in liquids. Studies on FC techniques made experimentally at the SAFL-UMN cavitation tunnel. FDB’s Jarle Ekanger’s PhD work included parts of these studies.

Impedance control by presence of free gas in pipe flows. FDB’s Ingrid Vilberg’s PhD work includes studies of the effect on hydropower water conduits dynamics performance as a function of gas content.

FDB’s research work currently receives partial funding from the Research Council of Norway, the Industrial PhD scheme, and the regional research fund for Rogaland, Hordaland and Sogn & Fjordane, RFF-Vest.